U.S. Pat. No. 4,337,201 (issued Jun. 29, 1982) describes inter alia the compound ##STR1## which has the generic name fosinopril and forms the alkali metal salt 1[S*(R*)],2.alpha.,4.beta.]-4-cyclo-hexyl-1-[[[2-methyl-1-(1-oxopropoxy)pr opoxy](4-phenylbutyl)phosphinyl[acetyl]-L-proline, monosodium salt. Fosinopril and related compounds are ACE inhibitors having antihypertensive activity and other utilities as described in U.S. Pat. No. 4,337,201.
Calorimetric, spectroscopic, and other analyses have established that fosinopril sodium exists as two polymorphs, A and B (see FIGS. 1 to 4). FIG. 1 shows a differential scanning calorimetry thermogram of the two polymorphs. FIG. 2 shows powder X-ray diffraction spectra of the two polymorphs. FIGS. 3A and 3B show infrared diffuse reflectance spectra of the polymorphs. FIG. 4 shows carbon-13 NMR spectra of the two polymorphs. Polymorph B has been found to be much more exothermic than polymorph A upon dissolving, so that polymorph A is believed to be more thermodynamically stable (see Table 1, below).